Piston for a hydraulic motor having radial pistons, and a method of manufacturing such a piston

ABSTRACT

A piston for a hydraulic motor including a body having a guiding and sealing surface, a base, and a top, which top is providing with a cradle-shaped recess. The base is provided with an additional cradle-shaped recess whose concave surface faces in the direction in which the concave surface of the recess in the top faces.

The present invention relates to a piston for a hydraulic motor havingradial pistons, said piston comprising a body having a guiding andsealing surface, a base, and a top, which top is provided with acradle-shaped recess.

The cradle-shaped recess in the top of the piston serves to receive aroller or wheel designed to roll on the cam of the radial-pistonhydraulic motor. In the meaning of the present invention, acradle-shaped recess has a concave surface that is substantially in theshape of a fraction of a cylinder.

While a radial-piston motor is operating, the cylinder block and the camof said motor move in rotation relative to each other, and the pistonsmove radially in reciprocating motion inside the cylinders of thecylinder block, their above-mentioned rollers rolling on the cam.

During this reciprocating motion, the guiding and sealing surface, whichis the side surface of the piston (generally in the shape of a cylinderhaving a base that is circular or of some other shape), co-operates inleaktight manner against the inside surface of the cylinder in which thepiston is received. The cylinder block and the cam moving in rotationrelative to each other, the piston is subject to tilt forces that arecompensated by the guiding due to the mutual co-operation of the twoabove-mentioned surfaces. The contact pressures between said surfacescan then be relatively high, and it is desirable to attempt to reducethem. In addition, the friction between the guiding surfaces of thepistons and the inside surfaces of the cylinders causes local heatingwhich, if it becomes excessive, can cause the piston to seize. The fluidpresent in the cylinder, between the end wall of the cylinder and thebase of the piston, limits the heating, in the regions in the immediatevicinity of the base of the piston. Unfortunately, in the portion of theguiding and sealing surface that is remote from said base, the coolingis not performed so well, and the risks of seizure can be high, inparticular in the middle regions of the guiding and sealing surfaces.

FR 2 648 512 discloses a hydraulic machine having radial pistons, inwhich machine each piston has a blind cylindrical hole at its base,centered on its axis of translation, the function of that hole not beingmentioned.

An object of the invention is to remedy the above-mentioned drawbacks,with a piston that, in addition, can be manufactured simply andinexpensively.

This object is achieved by the fact that the base is provided with anadditional cradle-shaped recess whose concave surface faces in thedirection opposite from the direction in which the concave surface ofthe recess in the top faces.

The recess has a concave surface that is substantially in the shape of afraction of a cylinder, and that is of area larger than the area of theflat end wall of the piston, so that the area of heat exchange with thefluid present in the cylinder is increased, thereby facilitating coolingof the piston. Another reason why this cooling is more effective thanthe cooling in the prior art is that the fluid present in the recess isat a radial level that is offset outwards relative to the inner end,thereby making it possible to cool zones of the guiding and sealingsurface that are somewhat remote from said inside end.

It should also be noted that the presence of the additional recesscauses material to be removed from the base of the piston, therebynaturally enabling the piston to be lightened.

Compared with the blind holes of FR 2 648 512, the additionalcradle-shaped recess, which is open at its ends situated in the guidingand sealing surface, enables the structure of the piston to be furtherlightened, enables the flexibility of the portions of the base of thepiston that flank the additional recess to be further increased, andenables cooling to be enhanced, by increasing the area of contactbetween the cylinder and the fluid present in said cylinder under thebase of the piston.

Advantageously, the two cradle-shaped recesses extend parallel to eachother.

As indicated above, the pistons are subjected to tilt forces relative totheir axes of translation, while the cylinder block and the cam arerotating relative to each other. The tilt forces are applied in planesperpendicular to the axis of relative rotation of the cylinder block andof the cam. The cradle-shaped recesses in the tops of the pistons extendparallel to that axis so as to enable the rollers present in saidrecesses to roll against the cam. In this situation, when the tworecesses of a piston extend parallel to each other, the additionalrecess is also parallel to the axis of relative rotation of the cylinderblock and of the cam. Because of the presence of the additional recess,the portions of the guiding and sealing surface that flank the recesspresent slight flexibility that enables them to deform in planesperpendicular to the axis of the recess. It is thus precisely in theplanes in which the tilt forces are high that said portions of theguiding and sealing surface can deform slightly and can thus increasethe mutually contacting surface areas. This makes it possible to limitthe negative impact of the tilt forces while reducing the contactpressure and while avoiding excessive localized friction.

The invention also provides a method of manufacturing pistons forradial-piston motors from a cylindrical bar having a longitudinal axis,each of said pistons comprising a body having a guiding and sealingsurface, a base, and a top, which top is provided with a cradle-shapedrecess.

In known methods, each piston is obtained from a billet, e.g. by cuttingup a cylindrical bar into segments. Each billet is drilled so as to formthe cradle-shaped recess in the top of the corresponding piston, whichrecess serves to receive the roller of the piston. In practice, acylindrical hole is drilled in an end portion of each billet, and,optionally, in order to form a stop surface inside the recess in the topof the piston, that hole is modified by means of a tool of the broachingtool type. In particular, said stop surface serves to retain acradle-shaped journal-bearing lining against the bottom of the recess,against which lining the roller of the piston rolls, which roller isalso retained in the recess. Then, that end portion is cut through thehole so that a portion of the hole, which portion defines thecradle-shaped recess, forms the top of the piston, while that end of thebillet in which the other portion of the hole was machined is discarded.That results in considerable wastage of material, it being possible forsuch wastage to be as high as about 50%.

An object of the invention is to improve that state of the art byproposing a method that makes it possible to achieve highermanufacturing rates and that limits wastage of material.

This object is achieved by the fact that the method consists inmachining cylindrical transverse holes at regular intervals in the bar,and, in order to form two adjacent pistons, in cutting up the bar on atransverse cutting-up plane passing through a hole, so that a firstportion of said hole forms a cradle-shaped recess in the base of thefirst one of the two adjacent pistons and so that another portion ofsaid hole forms a cradle-shaped recess in the top of the second one ofthe two adjacent pistons.

With the method of the invention, the holes are machined directly in thebar, prior to cutting up said bar, and they can thus be formedautomatically under excellent conditions. In particular, it is easier tohold the bar correctly over a long length, than to hold the individualbillets as in the prior art, while performing the hole-formingoperations. In addition, with the invention, a very significant savingin material is achieved because no hole is “wasted”, each hole having afirst portion that serves to form the recess in one piston, and a secondportion that serves to form the recess in the adjacent piston.

If machining (in particular broaching) is necessary for forming stopsurfaces in the recesses in the tops of the pistons, such machining isperformed in the bar that is provided with the series of holes, beforesaid bar is cut up.

In addition, each of the pistons obtained by means of this method hastwo recesses, thereby as indicated above, offering major advantages asregards limiting contact pressures, friction, and weight, and improvingcooling of the pistons equipping a radial-piston hydraulic motor.

The invention can be well understood and its advantages appear moreclearly on reading the following detailed description of an embodimentshown by way of non-limiting example. The description refers to thedrawings, in which:

FIG. 1 is a perspective view of a piston of the invention;

FIG. 2 is a section view on plane II of FIG. 1, containing the axis oftranslation of the piston, and perpendicular to the axis of its cradles;

FIG. 3 is a view analogous to FIG. 1, for a variant piston;

FIG. 4 is a view analogous to FIG. 2, for the piston of FIG. 3; and

FIGS. 5A to 5F show the manufacturing method of the invention.

The piston of FIG. 1 comprises a body 10 having a guiding and sealingsurface 12, a base 14 and a top 16.

The guiding and sealing surface is substantially in the shape of acylinder having a base that is circular or of some other shape, thecylindrical shape of the surface matching the shape of the cylinder inwhich the piston is designed to slide. The base of the piston is its endthat, when the piston is installed in the cylinder of a radial-pistonmotor, is closer to the end wall of the cylinder. The top of the pistonis opposite from its base.

It can be seen that the top 16 of the piston is provided with acradle-shaped recess 18. On the top of the piston, said recess forms aconcave surface 18A that is substantially in the shape of a fraction ofa cylinder of axis B perpendicular to the axis A of symmetry of thepiston, which axis of symmetry is the axis along which the piston isdesigned to move in translation in the cylinder block of a motor havingradial pistons.

The cradle-shaped recess 18 serves to receive a roller in the top of thepiston, which roller is designed to roll against the cam of theradial-piston motor. As can be seen more clearly in FIG. 2, thecradle-shaped recess 18 extends over greater than 180°, so that thedistance DE between the edges of the recess 18 is slightly less than thediameter D of the cylindrical surface for supporting said roller. In theexample shown, a lining 20 is received against the surface 18A of thecradle-shaped recess and is retained upwards by a shoulder 19 of therecess. The cylindrical surface for supporting the roller is defined bythe outside cylindrical surface 20A of the lining, which surface isflush with the portion of the recess 18 that extends between theshoulders 19 and the top edge of the piston.

The base 14 of the piston is provided with an additional cradle-shapedrecess 22 whose concave surface faces in the direction opposite to thedirection in which the concave surface of the recess 18 of the topfaces. It can be seen in FIG. 2 that the two cradle-shaped recesses 18and 22 extend parallel to each other. The recess 22 also forms a concavesurface that is substantially in the shape of a fraction of a cylinderof axis B′ parallel to the above-mentioned axis B. It can also be seenthat both of the axes B and B′ intersect the axis A of the piston, sothat the recesses 18 and 22 have a common plane of symmetry defined bythe axes A, B, and B′. Unlike the recess 18, the additional recess 22extends over less than 180°.

The surface 12 is provided with an annular groove 24 that is suitablefor receiving a sealing member such as a piston ring or gasket. It canbe seen in FIGS. 1 and 2 that the crest S22 of the cradle-shaped recess22 is situated in the vicinity of the groove 24, and indeed that saidrecess and said groove have a zone of intersection z.

In the example of FIGS. 3 and 4, the additional recess 221 is veryslightly shallower, and it can be seen that, although its crest S221also extends in the vicinity of the groove 24 in the surface 12 of thepiston, said crest does not intersect the groove.

Otherwise, the recesses 18 and 221 are analogous to the recesses 18 and22 of FIGS. 1 and 2, it being emphasized in particular that the concavecylindrical surface of the recess 18 extends over greater than 180°,unlike the concave surface of the recess 221. FIGS. 3 and 4 show apiston that has no lining, and whose recess receives the rollerdirectly. Preferably, and as shown in FIGS. 1 and 2, the cradle-shapedrecess 18 extends over greater than 180°, so that the distance DEbetween the edges of the recess 18 is slightly less than the diameter Dof the cylindrical surface for supporting said roller or wheel.

In reality, and as explained below with reference to the manufacturingmethod of the invention, the surfaces of the two recesses 18 & 22 or 18& 22′ are substantially complementary fractions of a completecylindrical surface, ignoring the setback in which the lining 20 isreceived, when such a setback is provided.

It is advantageous for the additional recess to extend to the vicinityof the groove 24, so as to lighten as much as possible the structure ofthe piston, the crests of the two recesses being relatively close toeach other. Depending on the type of sealing member used, the groove 24can have an intersection with the additional recess which, however, doesnot pass through it completely, or indeed, it can be preferable for suchan intersection not to exist.

The method of the invention is described below with reference to FIGS.5A to 5F. A starting bar 30 has a longitudinal axis L, and has acylindrical shape corresponding to the geometrical envelope of theguiding and sealing surface of the pistons that are to be formed fromsaid bar. Holes 34 extending transversely relative to the longitudinalaxis L are machined in said bar, the holes being disposed at regularintervals along the length of the bar and having a common plane ofsymmetry PS that contains the longitudinal axis L of the bar and theaxes B of the holes. For example, in order to machine the holes 34, aplurality of drill bits 32 are used that are spaced apart uniformly, orelse one drill bit is used that is moved along the bar after each holehas been formed.

In FIG. 5B, the bar has been turned through 90° relative to FIG. 5A, sothat it is possible to see the holes. The holes 34 are cylindrical. Inorder to form stop surfaces (shoulders 19) in the recesses 18 in thetops of the pistons, when such stop surfaces are desired, it is possibleto perform a specific machining step. This is shown in FIG. 5C in whichit can be seen that broaching tools 36 can be inserted into the holes 34so as to modify the inside surface locally.

In FIG. 5D, it can be seen that setbacks 35 in the shape of cylinderfractions have been formed in the holes by forming shoulders 19 thatextend parallel to the axes of the cylinders. For example, each of thesetbacks extends over about 180°.

As shown in FIG. 5F, sealing gasket grooves 24 are formed, e.g. byturning, in the cylindrical surface of the bar 30 (they can also beformed during the first operation for forming the bar), and separationzones 38 are also formed between pistons at regular intervals. Saidzones 38 are groove portions that are formed in the walls of the holes.They can be formed simultaneously with the grooves 24 or separatelytherefrom, by a milling operation. The separation grooves 38 generatezones of local fragility facilitating cutting up the bar into segmentsat said separation grooves. They also make it possible to bevel the topsof the pistons. It can be seen in FIG. 5E that each groove 38 occupies aplane PT that is transverse to the bar 30, which plane intersects a hole34. In order to form two adjacent pistons, the segment of bar is cut onsuch a plane PT so that a first portion 34A of the hole 34, whichportion is situated on a first side of the plane PT, serves to form acradle-shaped recess 22 in the base 14 of a piston P1, and so thatanother portion 34B of said hole, which portion is situated on the otherside of the plane PT, serves to form a recess 18 in the top of a pistonP2. The pistons P1 and P2 are adjacent while they are beingmanufactured, since they are defined on either side of a commoncutting-up plane PT.

Rectification operations can be performed on the pistons P1 and P2.However, it can be understood that the recess 22 of the piston P1corresponds almost entirely to the first portion 34A of the hole 34, andthat the recess 18 of the piston P2 corresponds almost entirely to thesecond portion 34B of said hole. In other words, the two recesses 18 and22 are two substantially complementary fractions of the completecylindrical surface of the hole 34, ignoring the setback 35, when saidsetback is present. By comparing FIGS. 5B and 5E, it can be seen thatthe transverse cutting-up plane PT is offset relative to the transverseplane of symmetry P34 of the hole 34 through which the plane PT passes.The plane P34 is transverse to the axis L of the bar and it contains theaxis B of the hole. Thus, when the bar is cut up on the plane PT, thetwo recesses coming from the hole 31 are of different sizes. The recess22 of the base of the piston P1 is the smaller of the two recesses.

In accordance with the invention, a plurality of piston manufacturingsteps are performed on the bar 30, before the individual pistons areseparated. Said bar can be initially cut to the desired length,compatible with the dimensioning of the machining machine. It can be abar segment of several tens of centimeters.

In particular, the operations of machining the holes 34 and the grooves38 and 24 are performed on the bar. In general, machining operationssuch as turning and milling are advantageously performed on the bar. Itis also possible to make provision for certain additional machiningoperations, e.g. rectification, treatment, coating, lining, orde-burring to be performed on the machined bar, before said machined baris cut up into individual pistons, with the advantage of reducing thehandling operations. This feature can even facilitate cutting up the barbecause certain treatments can increase the local fragility of theseparation zone in each groove 38, thereby facilitating cutting up. Forexample, the quantity of material defined between the end wall of thegroove 38 and the hole 34 can be small enough for cutting up to beobtained merely by applying a sufficiently violent impact.

1. A piston for a hydraulic motor having radial pistons, said pistoncomprising a body having a guiding and sealing surface, a top, which topis provided with a cradle-shaped recess, and a base which is providedwith an additional cradle-shaped recess whose concave surface faces inthe direction opposite from the direction in which the concave surfaceof the recess in the top faces.
 2. A piston according to claim 1,wherein the two cradle-shaped recesses extend parallel to each other. 3.A piston according to claim 2, wherein the cradle-shaped recesses have acommon plane of symmetry.
 4. A piston according to claim 1, wherein theguiding and sealing surface is provided with an annular groove suitablefor receiving a sealing member, of the additional recess is situated inthe vicinity of the groove.
 5. A piston according to claim 4, whereinthe additional recess and the groove have a zone of intersection.
 6. Apiston according to claim 1, wherein the surfaces of the two recessesare two substantially complementary fractions of a complete cylindricalsurface.
 7. A method of manufacturing pistons for a hydraulic motorhaving radial pistons, the method comprising the steps of: providinglongitudinal axis, machining cylindrical transverse holes at regularintervals in the bar, and, in order to form two adjacent pistons,cutting up the bar on a transverse cutting-up plane passing through ahole, so that a first portion (34A) of said hole forms a cradle-shapedrecess in the base of the first one of the two adjacent pistons and sothat another portion of said hole forms a cradle-shaped recess in a topof the second one of the two adjacent pistons.
 8. A method according toclaim 7, wherein the transverse cutting-up plane is offset relative tothe transverse plane of symmetry of the hole through which saidcutting-up plane passes so as to form two cradle-shaped recesses thatare of different sizes, the smaller recess being the recess in the baseof the first piston.
 9. A method according to claim 7, whereinadditional machining operations, such as rectification, treatment,de-burring, coating, lining, turning and milling are performed on thebar before it is cut up.